This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
An important aspect for the quality of experience perceived by an end-user watching television is the time needed to change channel (i.e. the zapping time). Naturally, the longer the end-user has to wait to access a new channel, the worse the experience is. However, contrary to what may be believed, despite—or perhaps because of—the technological advances, the zapping time is practically worse than ever.
In the early days of television, all that was needed to change channel was to turn or press a button, something that may be done very rapidly. Nowadays, though, the zapping time has become much longer, more for some technologies than for others. This is due to the accumulation of a number of small delays, each delay often being a side-effect of other, usually beneficial technical developments. Such delays comprise:                remote control transmission;        connection to the transport stream;        buffering of the incoming data;        Conditional Access (CA) system delays, such as acquisition of CA data and descrambler programming;        the time needed to find a first decodable image, such as a so-called I image for MPEG; and        audio synchronization.        
Technologies exist to reduce some of these delays, but the CA system has not been adequately addressed yet. The CA system can delay the channel change by up to one second. This delay is needed to acquire the CA data, decrypt it and program the descrambler. In many systems, the CA data is an Entitlement Control Message (ECM) that comprises an encrypted control word that the descrambler needs to access a scrambled program correctly.
FIG. 1 schematically illustrates a typical prior art CA system at the receiver end. The description is quite simplified and only describes features necessary for understanding the present invention. The receiver end 100 comprises a set-top box 110 that is connected to some kind of means for receiving 120, such as for example a parabolic antenna (as illustrated) or a cable. The set-top box 110 comprises a descrambler 140 and is also functionally connected to a security module 130, such as a detachable smartcard.
The receiving means 120 receives a scrambled program and encrypted ECMs for the scrambled program, each ECM comprising information enabling descrambling of the program during a time period called cryptoperiod (and possibly also during the subsequent cryptoperiod). The ECMs are sent 1 to the security module 130 and the scrambled program is sent 3 to the descrambler 140. The security module 130 decrypts the ECMs and forwards 2 extracted control words to the descrambler 140 that uses these to descramble the scrambled program so as to output 4 a program in the clear.
Often, the control word is changed about every 10 seconds, which means that the corresponding ECMs must be received so as to keep up. It is also common for an ECM to embed two control words, one for the current so-called crypto-period and one for the next. This provides some respite for the handling of the ECMs.
However, upon change of channel, the proper control word is not available as the ECMs are received only for the current channel. Thus, for a new channel, the set-top box usually has to wait for ECM extraction and decryption to occur before being able to descramble the program. This typically takes between 300 ms and 1 s, with 500 ms being a rough average value.
This delay may be better understood if it is known that that the security module and the set-top box have to perform a plurality of actions. The security module has to receive the ECM, decrypt the ECM, check the integrity of the ECM, verify the rights included in the ECM (e.g. verification the user has subscribed to the channel), encrypt and integrity protect the control word using a key shared with the set-top box, and send the protected control word to the set-top box. The set-top box has to extract the ECM (one ECM is typically present every 10 ms in the stream), send the ECM to the security module, receive the encrypted control word, decrypt the encrypted control word and verify the integrity of the control word.
In many current systems, this CA delay often does not cause any extra delays during zapping. The reason for this is shown in FIG. 2 that illustrates the commonly expected delays for the steps necessary to display a program after a change of channel in an exemplary Internet TV (IPTV) system.
After a channel change, 700 ms are needed for setting up the multicast. Only then can the buffering and the wait for the start of the Group Of Pictures (GOP) begin, which is performed in parallel with the reception of an ECM and extraction of the control word. When an I-Frame has been found, the hardware may descramble the scrambled program, which takes 100 ms, after which the MPEG4 decoding and display of the program takes another 700 ms. As can be seen from FIG. 2, the delay until the start of the GOP is expected to be 1000 ms, while the CA delay usually is shorter than that, such as an exemplary 500 ms. As these delays occur in parallel, it will be appreciated that the CA delay does not introduce any extra delay.
However, some solutions—such as for example those found in US 2003/0048808 and WO 2004/114668—reduce the duration of the delay for the GOP start so that it is shorter than the CA delay, while the solution described in European Patent Application 07301453.2 reduces the buffering delay.
It will thus be appreciated that there is a need for solutions that reduce the CA delay, in particular upon a channel change.
JP 2000101984 teaches a system in which the set-top box receives, from the transport stream, the ECM of all the channels for which the user has a subscription. The received ECMs are then stored. Thus, upon zapping, the set-top box immediately has the right ECM and can send it to the smart card for decryption. However, with such a solution, the CA delay is only marginally decreased. Typically, in a MPEG2-stream, an ECM is present every 10 ms. Thus, ECM extraction time is almost negligible compared with ECM decryption time.
As a variant in JP200101984 the ECMs are received by the smartcard that decrypts them. Only control words returned by the smartcard are stored by set-top box. This variant has one major drawback: since the ECM decryption time is typically between 300 ms and 1 second and cryptoperiods typically last 10 seconds (i.e. new ECMs every 10 s), the solution does not scale very well when it comes to the number of subscribed channels. If a bouquet includes more than 20 or 30 channels, the solution disclosed by JP200101984 cannot be applied, as the smartcard does not have the time to decrypt all the ECMs.
It can thus be appreciated that there is a need for an alternative solution that can enable reduction of the conditional access delay. The present invention provides such a solution, with a number of variants.